Spudgun Theory: Various relations will be derived and/or set forth as to the governing principles to the how and why to a spudgun's operation. Also some questions of 'why' are answered here. Some items of note to be included are the chemical equations for what you are burning (or trying to burn) in the combustion chamber, modeling the action/reaction of the potato and the launcher/wielder, calculating just how much pressure certain piping materials can withstand given environmental and physical factors, and modeling the effect of how a rotating object is stabilized (or destabilized) while traveling through our atmosphere.

My answer to one of the most frequently asked questions regarding combustion spudguns. (keep in mind that this is "theory" and individual results may vary)

How big should my chamber be??????

For combustion based spudguns.

Everyone wants to know what size to make their chamber. Or "How big is too big"? Or "How big can I make my chamber before it blows up on me"? Or "How can I get more distance out of my spudgun? I have a chamber that is 6" PVC pipe 48" inches long and it just doesn't seem to shoot very far".

For optimum performance the chamber should be 1 1/2 times the volume of the barrel. This is what is referred to as a 1.5:1 ratio. The reason for this is as follows. You want the spud to exit the barrel when combustion is almost complete, thus making use of all of the power created by the thermal expansion of the combustion gases. During the combustion, the chamber pressure will continue to increase until combustion is complete. Having a barrel that is sized to the correct ratio with the barrel allows the spud to keep gaining speed throughout the entire length of the barrel. Having a barrel that is too long or a chamber that is too small will cause the spud to slow down due to drag. If the potato has not exited the barrel by the time combustion has finished, the amount of pressure behind the spud begins to drop off very rapidly and in turn the spud is greatly affected by friction as it slides down the barrel without full pressure behind it. However, if the barrel is too short or the chamber is too large, the spud will be long gone before it has a chance to achieve maximum velocity. It would, in this case , only be making use of a small amount of the power from the thermal expansion gasses.

So in this respect, it's not the size that matters!!! And to answer the the question from the guy with the 6 x 48" chamber -- You would need a 2" barrel, 24 feet long to make it work the way it should. (not that I recommend building a chamber that size in the first place)

The volume of a piece of pipe can be found using the following formula.

Volume = Pi x radius sq. x length--(in inches)

Example: 12" piece of 4" ID pipe.

3.14 x 2 x 2 x 12 = 150.72 cubic inches of volume.

And for pneumatics.....

For a pneumatic launcher....There is no "ideal ratio" since everyone wants to accomplish different things.

The model tbl500lp, which has a 0.75:1 ratio will hit muzzle velosities of right around 300 fps. with a tennis ball.

For a good, high powered air cannon....A ratio of between 2:1 and 4:1 is generally a good bet. (such as the sp93xx) Anything above a 4:1 ratio hits the point of dimishing returns. You could build a launcher with an extreme ratio (say 15:1) and you certainly won't loose any power....But the amount of power you gain will not even be noticeable.

But at the same time...Amazing results can be achieved with relatively low ratios. Long barrels can do some pretty cool things. The big tri-pod mounted cannon on the "recent developments" page only has a ratio of 1.6:1, and I was able to brake the sound barrier with a potato with that obnoxious looking beast!!!

So in short......Build your pneumatic cannon as big as you want. If you have a big-ass air compressor...ROCK ON!!! If you are using a bike pump or a little 12v emergency compressor to charge up the chamber....you might want to keep it a bit smaller.

A little advice about fuel from visitors to the site.

(UPDATE:Right Guard no longer uses flamable propellent)

For those of us who don't want to take the plunge and go with propane, Right Guard is probably the best fuel that comes compressed in a can.

The original "Brown Can" variety commonly found in stores contains three (THREE!) different propellants: Alcohol denat, hydroflurocarbon 152A, and propylene glycol. After conducting side-by-side tests wherein I sprayed Aqua-Net and Right Guard over a lighter flame, I found that Right Guard burns a LOT more fully than Aqua-Net. The resulting flame is very large and hot. Aqua-Net, on the other hand, produced a relatively week flame, and the uncombusted portion was quite visible as it exited the flame zone. I didn't notice very much heat coming from it either, probably owing to the fact that so much of it didn't burn.

Right Guard isn't sticky, so it won't gum up the chamber. (Compare to hairsprays and other deodorants, which will coat your electrodes with goop and/or powder, giving you poor to zero ignition.)

The aforementioned brown can is larger than many hairspray cans, and the ratio of propellant to non-propellant is very high. You may notice a thin, wet film after combustion, but it isn't sticky or hard to clean off.

For these reasons, I think EVERYONE who is using the poor man's spud gun fuel should go to the store and buy some Right Guard forthwith!

Note that the type of Right Guard that doesn't come in a brown can probably has fragrances and other substances that will dilute the precious flammable portions of the formula, so GET THE BROWN CAN. Your spud gun doesn't need to smell flowery anyway.

cchant wrote Sept 3 2002:

So now you need to figure out how long your meter pipe needs to be. Calculate you chamber volume and and multiply it by 4% (0.04). This will give you the volume of propane you need at atmospheric pressure. However, the propane comes out of the tank around 110 to 130psi, so you'll have to convert that volume. Going back to chemistry, you know P1V1 = P2V2, where P1 is the initial pressure, V1 is the initial volume, P2 is the final pressure, V2 is the final volume. You would now be able to figure out the length of pipe needed at 130psi to correctly fill your chamber with 4% propane by volume.

Example calculation:

Your combustion chamber is 4"x12". The volume would be 150.72 cubic inches. Only 4% of that will be propane so multiply 150.72in^3 * 4% and you get 6.01in^3 of propane at atmospheric pressure.

P1V1 = P2V2

(130psi)V1 = (14.7psi)(6.01in^3) V1 = 0.68in^3

(14.7psi was converted from inHg which represents atmospheric pressure. This number can be found on a weather website for your local city.)

So you're meter pipe must be 0.68 cubic inches in volume, and say you went with 1/2" pipe for the meter, the length would be...

V = pi * radius^2 * length ...or... length = V / (pi * radius^2)

length = 0.68in^3 / (pi * 0.25in^2) length = 3.48in

So, with a 4"x12" chamber, you would need 3.48in of pipe between your ball valves (assuming 130psi) to get a perfect 4% propane mixture. Hope this was helpful.

Now here, the Excel mathematical model for how a pneumatic launcher works: Predicting the spud (projectile) velocity at the muzzle. Cuts up the barrel in 1000 little 'pieces' and calculates, quite rigorously, the velocity of the spud at the muzzle, using such variables as chamber volume, barrel size, spud mass, friction, and a few other things that you can enter into the program.